1. Field of the Invention
This invention pertains to barrier fluid seals, pumps and compressors, and more particularly to a barrier fluid seal assembly for a reciprocating rod, a reciprocating pump with the seal assembly and a method for operating reciprocating pumps.
2. Description of the Related Art
Moving shafts, such as reciprocating rods, frequently require a seal around the shaft. For example, reciprocating, plunger-type pumps and compressors require a seal around the plunger. Reciprocating pumps used in high-pressure applications, such as for chemical injection into a high-pressure vessel, have seals for containing a process fluid to be pumped, but it has been common to have some leakage past the seals. Such leakage is wasteful, and it leads to premature failure of the seals as well as being a source of pollution.
U.S. Pat. No. 5,746,435, issued to Arbuckle, discloses a dual seal barrier fluid leakage control method for a rotary pump. Arbuckle discloses a method for controlling leakage of a barrier fluid from a dual seal assembly for a rotary pump employing a barrier fluid supply arrangement connected to a barrier fluid chamber of the dual seal assembly. A pressure intensifier mechanism operates to maintain the pressure of the barrier fluid at a preset level above the pressure of a process fluid. An impeller rotates with a rotary drive shaft for circulating barrier fluid through a heat exchanger for cooling the barrier fluid. However, a reciprocating rod, pump or compressor does not have a rotary drive shaft for circulating barrier fluid.
A further problem with a reciprocating pump is that the discharge pressure spikes upward as the pump discharges process fluid and then falls off as a plunger within the pump retracts to repeat the pumping cycle. Unsteady cyclic pressure and flow adversely impacts process operations. U.S. Pat. No. 4,611,973, issued to Birdwell, discloses a mud pump comprised of plural cylinder pumping units, each cylinder unit consisting of a pumping compression chamber and two or more hydraulic driven expansion chambers. One or more expansion chambers are employed to drive a pumping plunger to cause fluid to be pumped through the compression chamber. This system is said to eliminate large pressure surges, but it allows small pressure surges. However, in some applications, such as very high-pressure chemical injection, such pressure surges are undesirable.